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This autumn is prime time for Jupiter-watchingBY MARK ARMSTRONGASTRONOMY NOWPosted: 6 October 2013

Jupiter is by far the largest planet in the Solar System and undoubtably the most rewarding planet to observe, image and study for the amateur astronomer.

Jupiter on September 3rd, 2013. Good seeing. Oval BA is central showing a notable orange colouration similar to last apparition. Some SSTC white ovals are passing it to the south. Turbulent activity within the NTB. Image and commentary by Damian Peach.

Despite its huge distance from us, its mean distance being more than five times farther from the Sun than we are, for most of the time it's the largest planet to view in a telescope, beaten out only by Venus when near inferior conjunction. Jupiter can get as large as 50 arcseconds in apparent equatorial diameter (it almost achieved this in September 2010 when it reached 49.9 arcseconds at a near-perihelic opposition) and varies between this and a minimum size of 43 arcseconds, with the mean being 46 arcseconds.

The face that Jupiter presents to us is not a solid surface like Mars or Mercury, but the cloud tops of its incredibly dynamic atmosphere with constantly changing features and detail within a familiar structure of long-lasting dark belts and bright zones that generations of astronomers have kept under constant surveillance. Jupiter's disc can be resolved even in binoculars and even a 50-mm class 'scope will show the two major equatorial belts and the polar regions. Employing a magnification of 150x will be sufficient to show lots of detail through the eyepiece high quality refractors in the 125-mm class and above and reflectors and SCT's in the 150mm- 200-mm class will reward the keen Jovian observer with a lifetime of enjoyment.

Great Red Spot

Jupiter's longest lasting feature is the famous Great Red Spot (GRS), a huge oval shaped, seemingly perpetual anticyclonic storm that has been observed since the 17th century. It varies in size and intensity and currently its strongly coloured brick-red, which makes it easier to see in small 'scopes. The GRS drifts constantly in longitude through the southern South Equatorial Belt (SEB) and can currently be found at System II longitude of 200 degrees. This drift amounts to as much as 12 degrees per year, and is, of course, independent of the exceedingly fast rotation of the planet itself.

there are two distinct rotation zones on Jupiter; the equatorial region, which encompasses the area from the southern half of the North Equatorial Belt (NEB) to the northern half of the SEB. Astronomers call this System I and has a defined mean rotation period of 9 hours 50 minutes and 30 seconds. System II is the rest of the planet and has a mean rotation period of 9 hours 55 minutes and 40.6 seconds. The GRS is best seen when it transits the Jovian central meridian (CM), an imaginary line drawn through the centre of Jupiter's disc from pole to pole.

Jupiter's fast rotation, as much as 90,000 km per hour at the equator, causes the globe to bulge outwards, giving the planet a very noticeable flattened appearance shaped like an oblate spheroid. This helps to define the CM easier and the system II longitude of the CM can be worked out in advance for any time, helping to pin-point a time when the GRS will be visible.

Oval BA is a smaller GRS-like feature that is at System II longitude of 40 degrees at the moment. Below are the times when the GRS and oval BA will transit the CM. Bear in mind that both will be visible for an hour or so either side of the times given, assuming the planet is high enough in the sky or twilight does not interfere.

Images show Jupiter as a very colourful planet but visually don't expect to see such vivid hues, although the larger the aperture of your telescope, assuming quality and clean optics and good quality eyepieces, the brighter Jupiter will be with more contrast, making cloud features easier to see and their colour easier to perceive.

Jupiter on September 4th, 2013. Fair seeing. The GRS is central and looking strongly coloured. The large white diffuse spot within the SEB just ahead of the GRS is well defined, though no circulatory structure is visible. Image and commentary by Damian Peach.

Colour filters will help the view through all telescopes but will really benefit smaller apertures. Red or orange filters will help enhance the bluer areas of Jupiter and help penetrate deeper into the atmosphere, while blue filters enhance the redder or darker areas. To enhance the contrast between the belts and zones, try a Wratten No. 80A or No.82A (light blue) and dark features such as festoons will benefit from a Wratten No. 12 (yellow) or No. 21 (orange). Other useful filters are No. 25 (red) and No. 38A (dark blue).

Galilean moons

The four Galilean moons of Jupiter, Io, Europa, Ganymede and Callisto, are another outstanding observation advantage that Jupiter has over the other planets, with their constant dance around their parent being a joy to follow and providing great observing opportunities. Binoculars are sufficient to see all four moons but perhaps a 150-mm is a good size instrument to comfortably observe all the satellite phenomena.

The satellites move from east to west across the face of the planet, and west to east behind it. They all orbit Jupiter in the same plane as Jupiter's equator so they will be in seen in transit, together with their shadows, across the face of Jupiter and seen to disappear and reappear behind Jupiter's limb (occultation). They also move in and out of Jupiter's shadow (eclipse). After conjunction with the Sun and before opposition, Jupiter's shadow falls to the west and eclipse precedes occultation and shadow-transit precedes transit. After opposition (next one January 2014) the order changes, occultation precedes eclipse and transits are before shadow-transits. All such events are predicted ahead of time and timings can be obtained in the Astronomical Almanac and the Handbooks of the British Astronomical Association and The Royal Astronomical Society of Canada.

Eclipses and reappearances are easy to see, the moon winking in and out of view, as are occultations. The dark shadows of the moons are readily observable in small to moderate 'scopes but the moons themselves can be tougher and generally require larger apertures and good seeing conditions. Volcanic Io, with its higher surface brightness, tends to be the easiest to spot. Transits can produce spectacular images. One important thing to note is that due to three degree tilt of Jupiter's equator to the plane of the ecliptic, the outermost moon, Callisto, appears to pass south of Jupiter and will not produce any phenomena events until after conjunction. Every now and again a beginner will casually glance at Jupiter through binoculars or a small scope and only see two or even one moon! These occurrences are not that uncommon as occasionally two moons will be in transit or one in transit and one hidden behind Jupiter or lost in its shadow.

Jupiter moons phenomena

Look out especially for events involving Callisto (IV), especially on the evening of 11/12 October when there is a rare triple shadow transit involving Io and Europa, too.

Jupiter is a splendid object in the morning sky among the stars of Gemini, rising but before midnight at the start of October. You will have to be a bit of a night owl to get your telescope on the gas giant at the earliest opportunity as the planet doesn't rise high enough until 2am BST. Jupiter rises before midnight and by 2am is 20 degrees up and ready for observation. At the start of nautical twilight when the Sun is 12 degrees below the horizon (5.48am BST) from London, Jupiter has attained a great altitude of 53 degrees. Jupiter has been closing in on delta Gem (mag. +3.5) all month and by the end of first week of October they are within a ten arcminutes. At this time Jupiter will be a brilliant mag. -2.26 and increased to 37.5 arcseconds in apparent diameter. Jupiter will not reach opposition in 2013 but very early next year, on 5 January.

Jupiter is very close to Wasat in October. This is the view in the small hours of October 5 when the separation between the two is a mere 10 arcseconds. By the end of October, Jupiter has stretched that out to 1.7 degrees. Graphic made using the Sky version 5.

In the months leading up to opposition, the planet will become more conspicuous in the morning sky, transiting the meridian at a very favourable 60 degrees at 5am (GMT) at the end of October. By November, Jupiter can start to be observed before midnight and has a nine hour observing window at the start of December.

High resolution images and detailed sketches of this new Jovian apparition are already flooding in and the good news is the the GRS is strongly coloured and oval BA still an obvious orange oval. Overall Jupiter has a 'normal' appearance, with the major belts and zones well defined. The Jupiter section of the British Astronomical Association has regular observing reports and updates; check out the first report of this new apparition. For the latest images go to the Jupiter section of ALPO-Japan. Astronomy Now magazine (available too on the iPad and iPhone) has all the latest news and images of Jupiter in the Night Sky section, with all the observing advice you will need to help you get the best out of your observing while the giant planet is so favourably placed.

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